The active duty list was a list of all Starfleet officers available for duty.
In 2267, though incapacitated, Captain Christopher Pike was still on the active duty list, as Starfleet, according to Commodore Mendez, "didn't have the heart to retire him".
Spock used this fact to his advantage when requesting a court martial, which required three commanding officers to officiate (in this case, James T. Kirk, Mendez, and Pike). (TOS: "The Menagerie, Part I")